Multiple access is a fundamental part of many communication systems having originating multipoint terminuses that operate autonomously. Such systems may be alternatively designated as multi-point-to-point systems. Point-to-multipoint systems, known as multiplex systems are the reverse of this. Multiplex systems may operate asynchronously while multiple access systems work best when synchronized.
In a multiple access communication system, multiple users typically share a common communication medium (or channel). A common example, of multiple access, is a wireless cellular communication system in which a single-base station communicates with a plurality of mobile subscriber units. To successfully achieve multiple access requires a plurality of substantially non-interfering channels to transmit the multifarious user information to be transmitted. Several well-established techniques exist in the art to achieve these non-interfering channels. The best known are frequency division multiple access (FDMA), time division multiple access (TDMA) and spread spectrum including direct sequence code division multiple access (DS-CDMA) and frequency hopping code division multiple access (FH-CDMA). These multiple access techniques have mostly been applied to wireless communication systems although their use is not limited to wireless systems.
FDMA is a technique of dividing an allocated frequency spectrum into a plurality of frequency bands with each individual frequency band supporting an information channel for a particular user. In many instances, N frequency bands serve as N information channels for N users.
TDMA divides a frequency band into a plurality of time slots wherein each user of that frequency band is assigned a particular time slot as that user's information channel. Information for a particular user is only transmitted during that user's designated time slot. A communication may operate with a plurality of frequency bands with each individual frequency band divided into a plurality of assignable time slots. This is sometimes known as a combination of TDMA and FDMA. TDMA normally requires a digital signal transmission since the information signals must be processed to fit into a particular time slot.
CDMA (Code Division Multiple Access) takes varied forms; the most common forms being DS-CDMA and FH-CDMA. DS-CDMA systems in pure form use a single common transmission frequency band that is shared in common by all users of that frequency band. Individual channels are achieved by having each user modulate/spread his particular information with a unique spreading code before transmission. The spreading code is re-applied at a receiver to recover or extract the particular information of that user. A TDMA and FDMA system may use the practices of CDMA within their defined channels.
FH-CDMA as a technique defines a particular user channel as a sequence of hopping frequency bands (i.e. a sequence of different frequency bands having a unique sequence of frequency band hops for each user). This is a technique in which no two successive bands, of the particular user channel, are identical and in which the frequency band hops are selected so that no two channels share a common frequency band at the same time.
In the realm of digital communications these techniques may further be categorized as having orthogonal or pseudo-orthogonal channels. In orthogonal access methods, the communication channel is divided into user channels that are non-interfering to one another. Pseudo-orthogonal channels allow some limited interference between user channels since the user channels are not perfectly orthogonal to one another. FDMA and TDMA are traditionally orthogonal while CDMA may be both. Orthogonality of communication system channels is an important consideration in designing a communication system.
Each of the techniques described above has its particular advantages and disadvantages, which are expressed in the nature of the information signals (i.e., analog, digital, etc.), the hardware apparatus (i.e., FM, AM, etc.), the robustness of varied signals (i.e., interference sensitivity, etc) and numerous other varied concerns. They are largely incompatible with one another. Selection of any one of these techniques implies a compromise between the advantages and disadvantages of each. It would be very advantageous to reduce the number of compromises required for any particular communication system requiring multiple access. It is critical that channel integrity be preserved in all available formats.
The piecemeal development of the foregoing prior art multiple access methods has led to incompatible multiple access methods. This has contributed to standards development that are inconsistent to one another. It further limits flexibility in achieving multiple access with particular goals to achieve.